First of all, the technique and principle of a conventional heterodyne displacement measuring interferometer will be explained hereinafter in order to improve the understanding of the present invention. The present technical situation of a laser light source of the heterodyne interferometer is described first. As implied by the word “heterodyne”, a laser light source of the heterodyne interferometer must output light having two frequencies which are at right angles to each other and linearly polarized, as shown in FIG. 1. There are two typical methods for generating this light. The first one is a method using the Zeeman effect, which applies a magnetic field to a gain material to generate two frequencies and the second one is a method using an acousto-optic modulator (hereinafter, referred to as “AOM”).
Specifically, the first method applies a magnetic field to a laser operating in a single mode using the Zeeman effect by which the spectrum of an atom is divided into two when a magnetic field is applied to the gain material, to thereby obtain light having two frequencies whose polarized beams are perpendicular to each other. This method can relatively easily construct a heterodyne light source. However, the maximum difference between the two frequencies is merely 3 MHz so that the maximum measuring speed is limited to 475 mm/s when a plane mirror interferometer is used.
The second method for obtaining a light source using an AOM became commercially available by Zygo, Co., of America. This method passes one of two frequencies of light that oscillate in a heterogeneous-mode laser through a polarizer and inputs the resultant frequencies to the AOM to obtain linearly polarized light having two frequency components with a frequency difference of 20 MHz between them. The light source obtained through this method has a high beat frequency so that a measuring band of the light source becomes larger than that of the light source acquired according to the Zeeman effect. Furthermore, even in the case where the beat frequency is stabilized and the laser is changed into another one, the beat frequency can be controlled to be identical to the stabilized one. However, the loss of light is considerable because one of the two frequencies should pass through the polarizer. Moreover, a polarization prism cannot perfectly align two beams with one axis, so measurement becomes difficult when a measuring distance is longer than 10 m.
Next, a phase measuring method and the technical situation of a conventional heterodyne interferometer will be explained. A phase measurer measures a frequency difference between a reference signal having a specific frequency and a measured signal obtained by adding the Doppler frequency to the frequency of the reference signal according to movement of an object and continuously adds up the measured frequency difference, to detect the displacement of the moving object. When a reflecting mirror moves at a rate v, Doppler frequency Δƒ according to the movement of the reflecting mirror is represented by the following Expression:Δƒ=2nv(t)/λ  (Expression 1)where n is the refractive index of air and λ is wavelength of light. When the reflecting mirror moves at 1 m/s, the Doppler frequency caused by the movement of the reflecting mirror is 3.16 MHz. In the case of a plane mirror interferometer where light reaches the reflecting mirror twice, the Doppler frequency becomes 6.32 MHz, which is twice the Doppler frequency of the above-described case. This means that 6.32×106 interference patterns pass for one second. The phase measurer must have a high frequency band so as not to miss the number of interference patterns.
The phase measurer mostly measures the ratio of the cycle of the reference signal to a zero crossing time difference of the reference signal and the measured signal, to obtain a phase value. The phase of the two signals is defined as follows:
                    ϕ        =                              360            °                    ×                                    T              +                                      T              R                                                          (                  Expression          ⁢                                          ⁢          2                )            where TR is the cycle of the reference signal VR, and T+ is the zero crossing time difference of the reference signal VR and the measured signal Vm. The phase measurer counts internal clocks during T+ and TR to obtain the phase value. Accordingly, the resolution of the phase measurer is determined by the cycle of the reference signal and a time resolution of the phase measurer. That is, the following relationship is accomplished:Δθ=360° ƒRΔτ  (Expression 3)where Δθ is an angle resolution of the phase measurer, ƒR is the frequency of the reference signal, and Δτ is the time resolution of the phase measurer. According to Expression 3, the phase resolution becomes 3.6+ when the time resolution of the phase measurer is 1 nsec and the frequency of the reference signal is 10 MHz, and the phase resolution is 0.36° when the frequency of the reference signal is 1 MHz. This means that the phase resolution increases as the frequency of the reference signal decreases. However, the phase resolution is limited by a measuring speed. In general, phase measurement is carried out for every one cycle of the reference signal so that the frequency of the reference signal becomes a sampling frequency of the phase value. When a phase variation is more than ⅔ during one sampling period, phase unwrapping becomes difficult. This can be represented by the following Expression:Δφ<|2π/3|  (Expression 4)where Δφ is a phase variation during a sampling period.
The limit of measurable Doppler frequency Δƒ can be obtained when both sides of Expression 4 are divided by sampling time, that is, half the cycle of the reference signal.
                                                    Δ            ⁢                                                  ⁢            f                                    =                                                        Δϕ                              2                ⁢                                  π                  ⁡                                      (                                                                  T                        R                                            2                                        )                                                                                            <                                    2              3                        ⁢                          f              R                                                          (                  Expression          ⁢                                          ⁢          5                )            where ƒR is the frequency of the reference signal, which is obtained through ƒ1−ƒ2.
In the case where the frequency of the reference is 1 MHz and the plane mirror interferometer is used, a maximum measurable movement speed is approximately 104 mm/s from Expressions 1 and 5. Because a maximum movement speed is proportional to the frequency of the reference signal, the frequency of the reference signal should be increased in order to measure an object moving rapidly. In this case, however, resolution is decreased. The relationship between the measuring speed v and a length resolution ΔL is represented as follows:
                    Δτ        <                  2          ⁢          Δ          ⁢                      L                          3              ⁢              v                                                          (                  Expression          ⁢                                          ⁢          6                )            
The conventional phase measuring method must satisfy the relationship of Expression 6. Accordingly, increasing the time resolution is the only way to improve both the measuring speed and the measuring resolution. To obtain the resolution of 0.3 mm at the movement speed of 2 m/s, a time resolution of at least 100 ps is needed. Thus, it is difficult and costly to obtain such resolution.
As described above, the conventional heterodyne interferometer has a large loss of light during generation of the light source so a light source with high power is difficult to obtain. In addition, the time resolution should be increased in order to acquire a high measuring speed and measuring resolution in the conventional heterodyne interferometer. However, there are limits in the time resolution, so it is difficult to actually construct a laser interferometer system.